A three-compartment bipolar membrane electrodialysis purification and lye regeneration system and method

Abstract

This invention belongs to the field of electrochemical separation and resource recovery technology, and discloses a three-chamber bipolar membrane electrodialysis purification and alkali regeneration system and method. The system includes a membrane stack composed of repeating units, each of which is sequentially arranged along the ion migration direction as a bipolar membrane, acid chamber, anion exchange membrane, salt chamber, cation exchange membrane, alkali chamber, and bipolar membrane. A mixture containing organic acid salts and an alkaline electrolyte is introduced into the salt chamber. Under the action of a DC electric field, the bipolar membrane dissociates water in situ into hydrogen ions and hydroxide ions. Organic acid ions migrate to the acid chamber to generate organic acids, while alkali metal cations migrate to the alkali chamber to generate regenerated alkali solution. The regenerated alkali solution is returned to the electrolytic cell for recycling. This invention eliminates the need for external acidification reagents and can simultaneously achieve organic acid purification and in-situ alkali regeneration under mild conditions. It has the advantages of high product purity, low energy consumption, no by-salt pollution, and suitability for industrial scale-up.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical separation and resource recovery technology, specifically relating to a three-chamber bipolar membrane electrodialysis purification and alkali regeneration system and method. Background Technology

[0002] In the field of hydrogen production through water electrolysis, the high overpotential of the oxygen evolution reaction (OER) at the anolyte is one of the key bottlenecks restricting the reduction of overall energy consumption. In recent years, auxiliary hydrogen production technologies that use the oxidation reaction of small organic molecules (such as methanol, ethanol, ethylene glycol, glycerol, etc.) to replace the traditional OER have attracted widespread attention. This process can not only significantly reduce the overpotential of the anolyte reaction and the operating voltage of the electrolyzer, but also generate organic acid salt products with added economic value on the anode side, such as formate, acetate, glycolate, lactate, or gluconate.

[0003] However, the aforementioned alcohol oxidation-assisted hydrogen production process is typically carried out in a strongly alkaline electrolyte (such as KOH, NaOH, or LiOH solution). The resulting organic acid salts are miscible with the highly concentrated alkaline electrolyte, forming a mixed system containing both organic acid salts and alkaline electrolyte. To achieve effective resource utilization of this mixed system, two technical requirements must be met simultaneously: first, the organic acid salts must be converted into free organic acids for subsequent separation and purification; second, the alkaline electrolyte must be recovered and recycled back to the electrolyzer to maintain continuous operation of the electrolysis process and reduce raw material costs.

[0004] The following technical approaches are currently mainly used to treat the aforementioned hybrid systems: (a) Conventional electrodialysis Conventional electrodialysis membrane stacks typically consist of alternating anion exchange membranes and cation exchange membranes, which migrate and concentrate anions and cations in a salt solution under the drive of a DC electric field. While this method can concentrate or desalinate organic acid salts when used to treat the aforementioned alkaline mixtures, it is limited by the lack of in-situ H₂ generation within the membrane stack structure. + and OH - The functional units of the electrolyte cannot directly convert organic acid salts into free organic acids. To obtain free organic acid products, an acidification step is still required after electrodialysis treatment.

[0005] (2) Chemical acidification method By adding inorganic acids such as sulfuric acid and hydrochloric acid to the mixture to adjust the pH of the system, organic acid salts are converted into free organic acids, which are then separated by extraction, crystallization, or distillation. While this method can achieve the acid conversion of organic acid salts, it has the following drawbacks: the introduction of inorganic acids leads to irreversible neutralization of the alkaline electrolyte, generating byproducts such as potassium sulfate and potassium chloride. This not only prevents the direct reuse of the alkaline solution but also increases the cost of waste salt disposal and the risk of secondary pollution. Furthermore, the consumption of the acidifying reagent itself increases operating costs.

[0006] (iii) Thermal evaporation or distillation concentration method This method utilizes the difference in boiling points between organic acid salts and alkali metal hydroxides to concentrate the mixture through heating and evaporation, causing the organic acid salts to crystallize or undergo further acidification. However, this method operates under high-temperature, strongly alkaline conditions, resulting in high energy consumption. Furthermore, some heat-sensitive organic acids in the alcohol oxidation products (such as glycolic acid and lactic acid) are prone to decomposition, coking, or polymerization side reactions under prolonged heating, leading to decreased product yield and purity. In addition, scaling and corrosion are significant problems when the evaporator operates in a strongly alkaline medium, affecting the long-term stable operation of the equipment.

[0007] In summary, existing technologies still face technical bottlenecks in simultaneously achieving efficient conversion of organic acid salts to free organic acids and in-situ recovery of alkaline electrolytes. Finding a way to convert organic acid salts to free organic acids under mild conditions without the addition of external acidifying agents, while simultaneously recovering high-purity alkaline electrolytes and achieving their cyclic coupling with the electrolytic cell, is a pressing technical challenge in this field. Summary of the Invention

[0008] The purpose of this invention is to provide a three-chamber bipolar membrane electrodialysis purification and alkali regeneration system and method, which solves the problems of difficulty in separating organic acid salts, dependence on external acid for the preparation of free organic acids, inability to recover alkali, high operating costs, and secondary salt pollution in the prior art.

[0009] This invention is achieved through the following technical solution: This invention discloses a three-compartment bipolar membrane electrodialysis purification and alkali regeneration system, comprising a three-compartment bipolar membrane electrodialysis membrane stack; The three-chamber bipolar membrane electrodialysis membrane stack is composed of at least one repeating unit, and each repeating unit includes, in sequence along the ion migration direction, a bipolar membrane, an acid chamber, an anion exchange membrane, a salt chamber, a cation exchange membrane, an alkali chamber, and a bipolar membrane. The salt chamber is used to introduce a mixed electrolyte containing organic acid salts and alkaline electrolyte; Under the action of a DC electric field, the bipolar membrane is used to dissociate water in situ into H+. + and OH - The organic acid anions in the salt chamber enter the acid chamber through the anion exchange membrane and react with the H+. + The alkali metal cations in the salt chamber combine to form organic acids, and then enter the alkali chamber through the cation exchange membrane and react with the OH-. - Combined with in-situ regenerated alkaline electrolyte, the regenerated alkaline electrolyte is returned to the electrolytic cell.

[0010] Furthermore, two adjacent repeating units share a single bipolar membrane, which serves as both the bipolar membrane at the end of the preceding repeating unit and the bipolar membrane at the beginning of the following repeating unit; or, each repeating unit independently has its own bipolar membrane at its beginning and end.

[0011] Furthermore, the organic acid salt is an organic acid salt produced by alcohol oxidation-assisted hydrogen production reaction, and is selected from one or more of formate, acetate, glycolate, lactate, gluconate, oxalate, and pyruvate; The alkaline electrolyte is one or more of KOH solution, NaOH solution, or LiOH solution, and the alkali metal cation is K. + Na + Or Li + One or more of them.

[0012] Furthermore, the bipolar membrane is composed of a cation exchange layer, an anion exchange layer, and an intermediate catalyst layer located between the two.

[0013] Furthermore, the acid chamber is provided with an acid inlet and an acid outlet. The acid inlet is used to introduce deionized water or dilute organic acid solution, and the acid outlet is connected to the organic acid collection unit. The alkali chamber is provided with an alkali inlet and an alkali outlet. The alkali inlet is used to introduce deionized water or alkali solution, and the alkali outlet is connected to the alkali return pipeline.

[0014] Furthermore, the salt chamber, acid chamber, and alkali chamber are each equipped with an independent circulation tank and circulation pump; The cation exchange membrane is a perfluorosulfonic acid membrane, a sulfonated polyether ether ketone membrane, a sulfonated polysulfone membrane, or other non-fluorinated cation exchange membranes. The anion exchange membrane is a quaternized polysulfone membrane, a quaternized polyether ether ketone membrane, a polybenzimidazole anion exchange membrane, or other alkali-resistant anion exchange membrane.

[0015] Furthermore, the membrane stack comprises 2 to 200 repeating units, and the thickness of the acid chamber, salt chamber, and alkali chamber of a single repeating unit is 1 to 10 mm, respectively.

[0016] Furthermore, it also includes one or more of the following: conductivity monitoring unit, pH monitoring unit, flow monitoring unit, temperature monitoring unit, or voltage monitoring unit.

[0017] This invention also discloses a three-compartment bipolar membrane electrodialysis purification and alkali regeneration method, comprising the following steps: (1) Pass the mixed electrolyte containing organic acid salts and alkaline electrolyte into the salt chamber; (2) Apply a DC electric field to the three-chamber bipolar membrane electrodialysis membrane stack, so that each of the bipolar membranes dissociates water in situ into H+. + and OH- ; (3) The organic acid anions in the salt chamber migrate through the anion exchange membrane to the acid chamber and react with the H+. + The combination generates organic acids, causing alkali metal cations in the salt chamber to migrate through the cation exchange membrane to the alkali chamber and react with the OH-. - This combines to generate a regenerated alkaline electrolyte; (4) Collect the organic acid products in the acid chamber and the regenerated alkaline electrolyte in the alkali chamber respectively, and return the regenerated alkaline electrolyte to the electrolytic cell.

[0018] Furthermore, the current density of the DC electric field is 50–400 A / m. 2 The operating temperature is 10–60 °C, the concentration of organic acid salts in the salt chamber feed is 0.1–5.0 mol / L, and the concentration of alkaline electrolyte is 0.1–5.0 mol / L.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects: This invention discloses a three-compartment bipolar membrane electrodialysis purification and alkali regeneration system, defining the basic structure of the three-compartment bipolar membrane electrodialysis stack. The repeating units, along the ion migration direction, are sequentially arranged as a bipolar membrane, acid chamber, anion exchange membrane, salt chamber, cation exchange membrane, alkali chamber, and bipolar membrane. Through this structure, organic acid anions and alkali metal cations in the salt chamber migrate directionally to the acid and alkali chambers respectively under the drive of an electric field, and react with H+ generated by in-situ water dissociation from the bipolar membrane. + and OH - By combining them separately, the conversion of organic acid salts to free organic acids and the in-situ regeneration of alkaline electrolyte can be completed simultaneously within the same membrane stack without the need for external acidifying reagents such as sulfuric acid, thus avoiding the generation of by-product salts and the waste of alkaline solution. At the same time, the regenerated alkaline solution can be directly returned to the electrolyzer to form a closed-loop circulation.

[0020] Furthermore, the bipolar membranes between adjacent repeating units can be configured either in a shared manner (N+1 membranes for N units) or independently (2N membranes for N units). Both methods provide flexible engineering options for system integration. The shared membrane method reduces membrane material usage and equipment investment costs, while the independent membrane method facilitates independent disassembly, replacement, and maintenance of individual repeating units, allowing for the selection of appropriate installation schemes based on actual engineering needs, membrane lifespan, and maintenance strategies.

[0021] Furthermore, the membrane stack is limited to 2–200 repeating units, with the thicknesses of the acid, salt, and alkali chambers in a single repeating unit ranging from 1 to 10 mm. This range covers laboratory-scale (2 units) to industrial-scale (200 units), and the chamber thickness is adjustable over a wide range. The number of repeating units can be linearly added according to the processing flux, demonstrating the system's good modularity and scalability, and facilitating flexible design of membrane stacks according to different production scales.

[0022] Furthermore, the system also includes one or more of the following: conductivity monitoring unit, pH monitoring unit, flow monitoring unit, temperature monitoring unit, or voltage monitoring unit. By collecting various operating parameters in real time, the membrane stack status and separation process can be determined online. Based on this, the feed flow rate, current density, or alkali reflux ratio can be dynamically adjusted to maintain the system in an optimal operating state, avoiding product quality degradation due to feed fluctuations or membrane performance decline, and improving the system's automation level and operational reliability.

[0023] This invention also discloses a three-chamber bipolar membrane electrodialysis purification and alkali regeneration method, comprising salt chamber feeding, applying an electric field to dissociate water from the bipolar membrane, directional ion migration to generate organic acid and regenerated alkali, product collection, and alkali reflux. This method closely integrates the membrane stack structure with the operational steps, clearly defining the complete path from raw material input to product output. In particular, directly connecting the alkali reflux to the electrolyzer to form a closed-loop cycle ensures the continuity and self-consistency of the entire process, achieving full-process separation from the alcohol oxidation mixed electrolyte to the pure organic acid product, while simultaneously ensuring the recycling of the alkaline electrolyte. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the closed-loop process of the three-chamber bipolar membrane electrodialysis purification and alkali regeneration system of the present invention coupled with the electrolytic cell; Figure 2 This is a schematic diagram of a single repeating unit of the three-compartment bipolar membrane electrodialysis membrane stack of the present invention, where BPM represents the bipolar membrane, AEM represents the anion exchange membrane, CEM represents the cation exchange membrane, and A... - M represents an organic acid anion. + Indicates an alkali metal cation; Figure 3 This is a schematic diagram showing the connection relationship between the pipelines and auxiliary units of the three-chamber bipolar membrane electrodialysis system of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0026] The components described and illustrated in the accompanying drawings and embodiments of this invention can be arranged and designed in various different configurations. Therefore, the detailed description of the embodiments of the invention provided in the following drawings is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the accompanying drawings and embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0027] like Figure 1 As shown, this invention provides a three-chamber bipolar membrane electrodialysis purification and alkali regeneration system, which can be coupled to an alcohol oxidation-assisted hydrogen production electrolyzer. The mixed electrolyte discharged from the anode side of the alcohol oxidation-assisted hydrogen production electrolyzer is pretreated and then fed into the three-chamber bipolar membrane electrodialysis stack. The organic acid solution produced by the three-chamber bipolar membrane electrodialysis stack is sent to the post-treatment purification unit, and the regenerated alkaline electrolyte is recycled back to the electrolyzer for reuse.

[0028] The three-chamber bipolar membrane electrodialysis stack consists of at least one repeating unit. Figure 2 The diagram illustrates the membrane arrangement of a repeating unit in a three-compartment bipolar membrane electrodialysis stack. Along the ion migration direction, the following are sequentially arranged: a bipolar membrane (BPM), an acid compartment, an anion exchange membrane (AEM), a salt compartment, a cation exchange membrane (CEM), an alkali compartment, and the bipolar membrane (BPM). Adjacent repeating units can share a single bipolar membrane (i.e., N+1 bipolar membranes for N repeating units) or each unit can be independently configured (i.e., 2N bipolar membranes for N repeating units). Both installation methods achieve the technical effects of this invention. Using a shared membrane reduces membrane material consumption and equipment costs, while using independent membranes facilitates independent disassembly and maintenance of individual units. In practical applications, the choice can be made flexibly according to engineering requirements.

[0029] In this invention, the cation exchange membrane can be selected from perfluorosulfonic acid membranes (such as the Nafion® series), sulfonated polyether ether ketone membranes, sulfonated polysulfone membranes, or other non-fluorinated cation exchange membranes with cation-selective permeability. Its function is to allow alkali metal cations (such as K+) to pass through. + Na + Li + Anion exchange membranes allow and block the passage of anions. Suitable membranes include quaternized polysulfone membranes, quaternized polyetheretherketone membranes, polybenzimidazole anion exchange membranes, and other alkali-resistant anion exchange membranes. Their function is to allow organic acid anions to pass through while blocking cations. All of the above membrane materials are commercially available, and those skilled in the art can select the appropriate type based on the pH, temperature, and ion species of the object being treated.

[0030] The bipolar membrane is composed of a cation exchange layer, an anion exchange layer, and an intermediate catalyst layer between them. The intermediate catalyst layer contains a hydrated hydrogen ion catalyst (such as metal oxide nanoparticles), which can significantly reduce the activation energy of water molecule dissociation under the action of an electric field.

[0031] Figure 3 The piping connections between the membrane stack and each auxiliary unit are shown. The salt chamber is connected to the mixed electrolyte storage tank via a feed pipeline and to the salt chamber circulation tank and circulation pump via a circulation pipeline, forming an independent circulation loop. The acid chamber is connected to the deionized water or dilute organic acid solution supply source via the acid inlet and to the organic acid collection unit via the acid outlet, and is equipped with an acid chamber circulation tank and circulation pump to form an independent circulation. The alkali chamber is connected to the deionized water or low-concentration alkali solution supply source via the alkali inlet and to the alkali return pipeline via the alkali outlet, and is equipped with an alkali chamber circulation tank and circulation pump to form an independent circulation. The independent circulation of each chamber can effectively reduce concentration polarization on the membrane surface, improve ion migration efficiency and product purity.

[0032] The three-compartment bipolar membrane electrodialysis stack has an anode and a cathode at each end, which are electrically connected to a DC power supply. When a DC electric field is applied, water dissociation occurs at the interface layer between the bipolar membranes: water molecules are catalytically dissociated into H+ under the influence of the electric field. + and OH - H + Driven by an electric field, OH- enters the acid chamber. - It enters the alkali chamber. Simultaneously, organic acid ions (A...) in the salt chamber... - Under the influence of an electric field, it migrates through the anion exchange membrane to the acid chamber, where it reacts with H+ in the acid chamber. + Combined to form free organic acids (HA); alkali metal cations (M) in the salt chamber + ) migrates through the cation exchange membrane to the alkaline chamber, where it reacts with OH- in the alkaline chamber. - This process generates a regenerated alkaline electrolyte (MOH). As organic acid anions and alkali metal cations continuously migrate out of the salt chamber, the salt concentration of the electrolyte in the salt chamber gradually decreases, thereby achieving the separation of organic acid salts and the simultaneous regeneration of the alkaline solution.

[0033] In actual operation, the current density and circulation flow rate can be adjusted according to the feed composition and throughput. When the concentration of organic acid salts in the salt chamber is high, the current density can be appropriately increased to increase the ion migration rate; when treating heat-sensitive organic acids (such as glycolic acid, lactic acid, etc.), the operating temperature should be controlled within a low range to avoid product decomposition. The circulation flow rate of each chamber is adjusted by a circulation pump, and the circulation velocity of each chamber is generally controlled at 5–20 cm / s to ensure that the fluid on the membrane surface is in a turbulent state and to reduce the thickness of the concentration polarization layer.

[0034] The system also includes one or more of the following: conductivity monitoring unit, pH monitoring unit, flow monitoring unit, temperature monitoring unit, or voltage monitoring unit. During operation, parameters such as conductivity, pH, temperature, flow rate, and membrane stack voltage of each chamber can be collected in real time through each monitoring unit. Based on this, the feed flow rate, current density, or alkali reflux ratio can be adjusted to maintain the system in an optimal operating state.

[0035] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0036] Example 1 This embodiment uses a mixed electrolyte containing 1.0 mol / L potassium glycolate (an oxidation product of ethylene glycol) and 1.0 mol / L KOH as the treatment target. The three-compartment bipolar membrane electrodialysis stack consists of 10 repeating units, each arranged as follows: bipolar membrane, acid compartment, anion exchange membrane, salt compartment, cation exchange membrane, and alkali compartment. Each of the acid, salt, and alkali compartments has an independent circulation loop.

[0037] The device parameters are: processing throughput 100 m³ / s. 3 / h; Current density 200 A / m 2 Operating temperature: 30 ℃; Effective area of ​​a single membrane: 0.5 m² 2 / unit; the thickness of the acid chamber, salt chamber, and alkali chamber is 4 mm.

[0038] During operation, a mixed electrolyte containing salt is continuously passed into the salt chamber, while deionized water or a dilute organic acid solution is used as the initial circulating liquid in the acid chamber, and a low-concentration KOH solution (0.05 mol / L) is used as the initial circulating liquid in the alkali chamber. After applying a DC electric field, glycolate ions migrate to the acid chamber to form glycolic acid. + Migrate to the alkali chamber and react with OH - The generated KOH was then combined. The system ran continuously for 24 hours, with samples taken for analysis every 2 hours. The test results are shown in Table 1.

[0039] Table 1

[0040] Table 1 shows that the system of this invention can convert potassium glycolate into high-purity glycolic acid without the addition of external chemical acidifiers, while simultaneously regenerating KOH in situ. The regenerated KOH can be returned to the alcohol oxidation-assisted hydrogen production electrolyzer, thereby reducing the amount of fresh alkali solution to be added. No obvious abnormal membrane stack pressure drop or rapid decay of product purity was observed during continuous operation, indicating that the structure has good operational stability.

[0041] Based on theoretical annual capacity estimates, the system capital expenditure (CAPEX) for this embodiment is approximately 20 MEUR, the annual operating expenditure (OPEX) is approximately 14.2 MEUR, the investment payback period is approximately 11 years, and the cost per ton of product is approximately 285 EUR / t. This economic calculation is used to illustrate the cost advantage of this invention compared to chemical acidification and thermal evaporation methods; actual values ​​can be adjusted based on membrane stack size, electricity price, membrane lifetime, and product price.

[0042] Example 2 This example was used to verify the adaptability of different organic acid salt systems. Mixed electrolytes were prepared using 1.0 mol / L potassium formate, 1.0 mol / L potassium acetate, 1.0 mol / L potassium lactate, and 1.0 mol / L KOH, respectively. The membrane stack structure and main operating parameters were the same as in Example 1. The test results are shown in Table 2.

[0043] Table 2

[0044] Table 2 shows that the system of the present invention has good adaptability to organic acid salts with different carbon chain lengths and structures. Compared with formate and acetate, lactate has a larger molecular weight and slightly lower diffusion and migration rate, resulting in a slight decrease in current efficiency, but the purity of organic acid and the alkali regeneration rate still meet the requirements for industrial applications.

[0045] Those skilled in the art will understand that when LiOH is used as an alkaline electrolyte, due to the presence of Li... + hydrated ionic radius and K + Na + Similarly, their migration behavior in cation exchange membranes is similar, and they can also achieve the separation of organic acid salts and the regeneration of alkaline solutions. Therefore, this invention is also applicable to lithium-based alkaline electrolyte systems.

[0046] Example 3 This example illustrates the effect of current density on separation performance. With potassium glycolate concentration fixed at 1.0 mol / L and KOH concentration fixed at 1.0 mol / L, a current density of 100 A / m was set. 2 200 A / m 2 300 A / m 2 and 400 A / m 2 The remaining conditions were the same as in Example 1. The test results are shown in Table 3.

[0047] Table 3

[0048] Table 3 shows that lower current densities result in higher product purity, alkali regeneration rate, and current efficiency, but lower throughput per unit time. Excessively high current densities lead to increased water migration, ion back-diffusion, and side reactions, resulting in decreased current efficiency and increased energy consumption. Therefore, 100–300 A / m² is a suitable range, with approximately 200 A / m² balancing processing efficiency, product quality, and energy consumption.

[0049] Comparative Example 1: Conventional Electrodialysis This comparative example uses a conventional electrodialysis membrane stack without bipolar membranes. The membrane stack includes an anode chamber, anion exchange membrane, concentration chamber, cation exchange membrane, desalination chamber, and cathode chamber, or a concentration / desalination channel formed by alternating anion exchange membranes and cation exchange membranes. A mixed electrolyte containing 1.0 mol / L potassium glycolate and 1.0 mol / L KOH is passed into the desalination channel or feed channel at 200 A / m. 2 The test was conducted for 24 hours under the specified conditions. The test results are shown in Table 4.

[0050] Table 4

[0051] Conventional electrodialysis mainly relies on the migration of anions and cations to achieve salt concentration or desalination, and cannot generate H+ in situ. + and OH - Therefore, organic acid salts cannot be directly converted into free organic acids. If organic acids are subsequently obtained, they still need to be acidified with inorganic acids, leading to by-salt contamination and alkali loss.

[0052] Comparative Example 2: Chemical Acidification + Extraction Method Take 1 L of a mixed electrolyte containing 1.0 mol / L potassium glycolate and 1.0 mol / L KOH, and slowly add sulfuric acid under stirring to adjust the pH to 2.5 to release glycolic acid. Then, extract with ethyl acetate and recover the glycolic acid by vacuum distillation. The test results are shown in Table 5.

[0053] Table 5

[0054] While chemical acidification can convert organic acid salts into free organic acids, it consumes inorganic acids and generates a large amount of by-salts. At the same time, KOH is irreversibly neutralized and cannot be returned to the electrolytic cell for use. The loss and recovery of extractant also increase the complexity of operation.

[0055] Comparative Example 3: Thermal Evaporation Concentration Method Rotary evaporation was used to simulate the thermal evaporation concentration process. Under a vacuum of 0.09 MPa and a temperature of 80 °C, 1 L of a mixture containing 1.0 mol / L potassium glycolate and 1.0 mol / L KOH was concentrated to 200 mL, cooled and crystallized, and then acidified to obtain glycolic acid. The test results are shown in Table 6.

[0056] Table 6

[0057] Thermal evaporation is prone to decomposition and coking of heat-sensitive organic acids or intermediates under strong alkaline and high-temperature conditions, and leads to scaling of the evaporator. Its energy consumption is significantly higher than that of the electrically driven membrane separation process of this invention.

[0058] Industrial Application Examples A factory uses an ethylene glycol oxidation-assisted hydrogen production process to process 10 tons of ethylene glycol per day, producing approximately 200 m³ of mixed electrolyte containing approximately 2.0 mol / L potassium glycolate and approximately 1.5 mol / L KOH. 3 / d. The three-compartment bipolar membrane electrodialysis system of this invention was modified, with the membrane stack containing 100 repeating units and a single membrane effective area of ​​2.0 m². 2 / unit, the installation method adopts the shared bipolar membrane of adjacent units (N+1 mode). After the system is put into operation, the regenerated KOH produced in the alkali chamber is continuously returned to the electrolyzer through the reflux pipeline, and the glycolic acid solution produced in the acid chamber is sent to the subsequent purification unit for further concentration and crystallization to obtain glycolic acid product. The operating indicators are shown in Table 7.

[0059] Table 7

[0060] The operational indicators after the upgrade are as follows: daily glycolic acid production is approximately 9.2 t, with a purity of approximately 97%; daily KOH replenishment is only about 0.2 t / d (only replenishing system mechanical losses and a small amount of alkali carried out with the product), a reduction of approximately 95% compared to the 4.5 t / d purchased amount of the chemical acidification method before the upgrade; sulfuric acid consumption and waste salt production are reduced to zero; wastewater discharge is reduced from approximately 180 m³ / d. 3 / d drops to approximately 20 m 3 The cost per ton of product processed decreased from approximately 456 EUR / t before the modification to approximately 285 EUR / t. A comparison before and after the modification shows that the system of this invention can achieve synergistic operation of organic acid purification and alkali regeneration, significantly reducing reagent consumption, waste salt discharge, and overall operating costs. Furthermore, the membrane stack pressure drop did not show a significant increase during three months of continuous operation, indicating that the system has good engineering stability.

[0061] It should be noted that the thickness of the acid, salt, and alkali chambers can be adjusted within the range of 1–10 mm depending on the throughput and membrane stack size. Thinner chambers (e.g., 1–3 mm) result in shorter inter-membrane distances and shorter ion migration paths, which helps reduce membrane stack resistance and energy consumption, but the throughput decreases accordingly. Thicker chambers (e.g., 7–10 mm) increase the throughput per chamber, but the resistance increases slightly. The selection within the above thickness range is a conventional optimization method in this field, and those skilled in the art can determine the optimal thickness within the 1–10 mm range through a limited number of experiments based on the actual throughput.

[0062] Those skilled in the art will understand that the concentration of organic acid salts in the salt chamber feed can be adjusted within the range of 0.1–5.0 mol / L according to the production conditions of the upstream electrolyzer. At lower concentrations (0.1–1.0 mol / L), appropriately reducing the current density can maintain a high desalination rate and current efficiency; at higher concentrations (3.0–5.0 mol / L), appropriately increasing the current density and increasing the circulation flow rate can ensure sufficient ion migration and acid-base formation rates.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A three-chamber bipolar membrane electrodialysis purification and alkali regeneration system, characterized in that, Including a three-compartment bipolar membrane electrodialysis membrane stack; The three-chamber bipolar membrane electrodialysis membrane stack is composed of at least one repeating unit, and each repeating unit includes, in sequence along the ion migration direction, a bipolar membrane, an acid chamber, an anion exchange membrane, a salt chamber, a cation exchange membrane, an alkali chamber, and a bipolar membrane. The salt chamber is used to introduce a mixed electrolyte containing organic acid salts and alkaline electrolyte; Under the action of a DC electric field, the bipolar membrane is used to dissociate water in situ into H+. + and OH - The organic acid anions in the salt chamber enter the acid chamber through the anion exchange membrane and react with the H+. + The alkali metal cations in the salt chamber combine to form organic acids, and then enter the alkali chamber through the cation exchange membrane and react with the OH-. - Combined with in-situ regenerated alkaline electrolyte, the regenerated alkaline electrolyte is returned to the electrolytic cell.

2. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, Two adjacent repeating units share a single bipolar membrane, which serves as both the bipolar membrane at the end of the preceding repeating unit and the bipolar membrane at the beginning of the following repeating unit; or, each repeating unit independently has its own bipolar membrane at its beginning and end.

3. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, The organic acid salt is an organic acid salt produced by alcohol oxidation-assisted hydrogen production reaction, and is selected from one or more of formate, acetate, glycolate, lactate, gluconate, oxalate, and pyruvate; The alkaline electrolyte is one or more of KOH solution, NaOH solution, or LiOH solution, and the alkali metal cation is K. + Na + Or Li + One or more of them.

4. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, The bipolar membrane is composed of a cation exchange layer, an anion exchange layer, and an intermediate catalyst layer located between the two.

5. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, The acid chamber is provided with an acid inlet and an acid outlet. The acid inlet is used to introduce deionized water or dilute organic acid solution, and the acid outlet is connected to the organic acid collection unit. The alkali chamber is provided with an alkali inlet and an alkali outlet. The alkali inlet is used to introduce deionized water or alkali solution, and the alkali outlet is connected to the alkali return pipeline.

6. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, The salt chamber, acid chamber, and alkali chamber are each equipped with an independent circulation tank and circulation pump; The cation exchange membrane is a perfluorosulfonic acid membrane, a sulfonated polyether ether ketone membrane, a sulfonated polysulfone membrane, or other non-fluorinated cation exchange membranes. The anion exchange membrane is a quaternized polysulfone membrane, a quaternized polyether ether ketone membrane, a polybenzimidazole anion exchange membrane, or other alkali-resistant anion exchange membrane.

7. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, The membrane stack comprises 2 to 200 repeating units, and the thickness of the acid chamber, salt chamber, and alkali chamber of a single repeating unit is 1 to 10 mm, respectively.

8. The three-chamber bipolar membrane electrodialysis purification and alkali regeneration system according to claim 1, characterized in that, It also includes one or more of the following: conductivity monitoring unit, pH monitoring unit, flow monitoring unit, temperature monitoring unit, or voltage monitoring unit.

9. A method for purification and alkali regeneration via three-compartment bipolar membrane electrodialysis, characterized in that, The system according to any one of claims 1 to 8 comprises the following steps: (1) Pass the mixed electrolyte containing organic acid salts and alkaline electrolyte into the salt chamber; (2) Apply a DC electric field to the three-chamber bipolar membrane electrodialysis membrane stack, so that each of the bipolar membranes dissociates water in situ into H+. + and OH - ; (3) The organic acid anions in the salt chamber migrate through the anion exchange membrane to the acid chamber and react with the H+. + The combination generates organic acids, causing alkali metal cations in the salt chamber to migrate through the cation exchange membrane to the alkali chamber and react with the OH-. - This combines to generate a regenerated alkaline electrolyte; (4) Collect the organic acid products in the acid chamber and the regenerated alkaline electrolyte in the alkali chamber respectively, and return the regenerated alkaline electrolyte to the electrolytic cell.

10. The method according to claim 9, characterized in that, The current density of the DC electric field is 50–400 A / m. 2 The operating temperature is 10–60 °C, the concentration of organic acid salts in the salt chamber feed is 0.1–5.0 mol / L, and the concentration of alkaline electrolyte is 0.1–5.0 mol / L.

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